A Micro-Electro-Mechanical Systems (MEMS) device is typically enclosed in a sensor block assembly (SBA) to facilitate mounting of the MEMS device in its application device. For example, a MEMS gyroscope or accelerometer may be used to determine angular rotation or acceleration, respectively, of the installation vehicle in which it is installed.
The MEMS device may be subject to severe temperature excursions when the application device is operating in the field. For example, if the application device is a satellite or space craft, the satellite or space craft will be subjected to extreme temperature variations.
To prevent the MEMS device from exposure to cold temperatures, a heater may be physically coupled to, or implemented within, the sensor block assembly. As temperature of the application device decreases, the heater will operate to maintain temperature of the MEMS device within a predefined temperature threshold. The temperature control system of the heater relies on an external power source, such as a battery.
Various design considerations may, at times, impose limits on the amount of power and/or current that may be drawn by the heater. For example, if the power source is a battery, the total deliverable power deliverable from the battery may be limited based upon some criteria, such as a designed operating period between battery charging. Accordingly, a power budget may be allocated to an individual heater so that at any given time, the power draw from that heater is limited to a specified power budget.
Further, design considerations may limit the amount of root mean square (RMS) current drawn by the heater. Since a heater is typically a resistive device, the generated heat will decrease as the power supply voltage decreases. To compensate, a pulse width modulation (PWM) controller may increase the duty factor, and thus increase input current, to maintain a constant delivered power. However, at some point, a specified RMS current limit may be reached and thus limit the power to the resistive load.
Accordingly, it is desirable to improve the ability of the heater control system to respond to changes in operating temperature by closely adhering to a specified power draw budget and/or a specified current limit independent of the power source voltage and independent of any average current limit.